The use of relatively high power pump light (e.g., about 500 W or more) within fiber-based lasers and amplifiers has introduced a new problem in the output efficiency of these devices. In particular, modal instability has been found to occur in these high power systems, where in this case modal instability takes the form of oscillations between the fundamental mode (LP01) and a higher-order mode (typically the LP11 mode) in the output signal from the gain fiber. Certainly, this modal instability is undesirable in high power applications that require a steady, stable output beam.
The modal interplay is facilitated by the elevated temperatures reached in the core region of a gain fiber during high power operation, which is attributed to quantum defect heating as pump light is converted into signal light within the rare-earth doped core of the gain fiber. A key to mitigating modal instability is to reduce the growth of power within the unwanted higher-order modes (HOMs), or to reduce the core temperature of the gain fiber, or both.
One way to reduce the possibility of modal instability is to increase the power loss (or reduce the gain) within the HOM beam. For example, it is possible to dope only a portion of the gain fiber, thus reducing the amount of gain introduced into the unwanted HOM beam. The obvious drawback of this approach is that a longer length of gain fiber is needed for complete pump absorption. Longer lengths of fiber are known to introduce nonlinear effects into the amplified optical signal and need to be avoided. Instead of doping only a portion of the gain fiber, it is possible to reduce the dopant concentration along the length of the fiber, reducing the core temperature of the gain fiber. This solution has the same undesirable result of also needing a relatively long length of gain fiber to achieve complete (or nearly complete) absorption of the pump beam.
Thus, a need remains in the art for a gain fiber configuration that is able to work at relatively high power levels without exhibiting modal instability. Said another way, the need remains for raising the power threshold at which modal stability occurs to a level beyond that currently utilized for fiber laser/amplifier systems.